The present invention relates to a pulse width modulation circuit (referred to below as PWM circuit) for modulating pulse width, more specially the invention relates to a PWM circuit that modulate pulse width at low frequency.
FIG. 9 is a diagram showing an example of related PWM circuit. This circuit comprises a triangle wave generator with an input of oscillation signal (referred to below as OSC) and a comparator C1, the input of which is the output signal (node1) of the triangle wave generator (referred to below as TWG) and a signal Vin. FIG. 10 shows an example circuit of the TWG.
The operation of the circuit of FIG. 9 is described below with reference to the FIG. 11. The TWG outputs a triangle wave signal on node1 according to the “high” and “low” level of input signal OSC. The comparator C1 compares the level of input signal Vin at non-inverting input terminal and the level of input signal of node1 at inverting input terminal, and outputs “high” level signal or “low” level signal according to the comparison result.
To be more precise, when OSC signal level is “high”, the signal level of node1 decreases gradually and increases gradually at the “low” level section of OSC signal. The triangle wave signal is generated with the repetition of this operation.
The comparator C1 outputs “high” level signal when the level of the output signal of the TWG is lower than that of input signal Vin, thus “high” level output section occupies, for example, about 40% per one cycle of OSC signal (Duty=40%). This is the case of lower level input of the signal Vin. On the other hand, in case of higher input level of Vin, “high” level output section is extended, so the “high” level section of the output signal “OUT” of the comparator C1 is reached to, for example, about 70% (Duty=70%).
In this way, a pulse width modulation of the output signal “OUT” is possible according to the input level of the signal Vin. However, the circuit of FIG. 9 must operate in the period “OSC”, so faster response speed is needed for the comparator used in PWM circuit and operational amplifier used in the TWG. This leads to the drawback of increasing power consumption. In addition, the comparator operates in response to the level sifting of the input Vin, so wider input range operability is needed for the comparator. This leads to the drawback of increasing number of construction elements, increasing layout size, increasing power consumption. In addition, as the input level of Vin is shifted, so the common mode input characteristic of the comparator C1 deteriorates and a large offset voltage occurs according to the level of Vin when a supply voltage goes down. This leads to the drawback of decreasing the low voltage operation margin.